undo

undo is an interactive compiled Lisp function in `simple.el'.

It is bound to C-_, <undo>, C-/, C-x u, <menu-bar> <edit> <undo>.

(undo &optional ARG)

Undo some previous changes.
Repeat this command to undo more changes.
A numeric ARG serves as a repeat count.

In Transient Mark mode when the mark is active, only undo changes within
the current region. Similarly, when not in Transient Mark mode, just C-u
as an argument limits undo to changes within the current region.